1. Field of the Invention
The present invention relates to an active antivibration device on an antivibration target member and capable of demonstrating an active antivibration effect by vibrating a mobile element by applying a current to a coil, and to a manufacturing method of the active antivibration device.
2. Description of Related Art
For example, in order to reduce vibrations of an antivibration target member which is required to reduce vibrations such as a body of a vehicle, generally, a vibration attenuating mechanism utilizing an attenuation effect of, for example, a shock absorber and a rubber elastic body, or a vibration isolating mechanism utilizing a spring effect of, for example, a coil spring and the rubber elastic body has been adopted. These vibration attenuating mechanism and vibration isolating mechanism demonstrate only a passive antivibration effect. Therefore, recently, a mechanism demonstrating a more active and higher antivibration effect has been desired.
With respect to this kind of active antivibration device, for example, Japanese Patent Publication No. 2008-208895 discloses an active antivibration device where a concentric positioning between an output member and a fixed side supporting member is accurately performed, thereby a clearance between a coil and a permanent magnet is accurately set, and as a result, the output member can be effectively vibrated.
Meanwhile, in the active antivibration device disclosed in Japanese Patent Publication No. 2008-208895, the output member on a mobile side includes a core metal having a rod shape and movably supported for applied vibrations along an axial direction at the center of the device body, an inner yoke engaged with the core metal from the outside, an outer yoke having substantially a cylindrical shape and connected to an outer side of the inner yoke with a predetermined clearance, and a cylindrical permanent magnet which is disposed between the inner yoke and the outer yoke along a circumferential direction of the inner yoke and the outer yoke and mounted on an annular step of the inner yoke. Those of the core metal, the inner yoke, the outer yoke, and the permanent magnet are disposed to be movable in a unified manner along the up and down direction. On the other hand, with respect to the fixed side, a structure that a coil arranged between the permanent magnet and the outer yoke is supported by the device body is adopted.
In this case, the cylindrical permanent magnet engaged with the inner yoke from the outside is magnetized in the radial direction of the cylindrical permanent magnet, where magnetic poles at the inner surface and at the outer surface of the permanent magnet are different with each other. Then, there is a possibility that a magnetic saturation is caused in the inner yoke arranged on the inner side of the permanent magnet by a combination of a magnetic flux generated by the permanent magnet and a magnetic flux generated by the coil by applying a current. As a result, a magnetic loss in the magnetic flux path increases, and an improvement of a magnetic propulsion force, that is, the improvement of a vibration applying force to be given to the mobile element becomes difficult.
In addition, in the active antivibration device disclosed in Japanese Patent Publication No. 2008-208895, an inner portion of a disc spring member is assembled first against a mobile axis portion which constitutes the output member in the assembly process. Therefore, an outer periphery portion of the disc spring member is required to be fixed by, for example, a securing ring fixed to an inner wall of the housing after the mobile axis portion is housed inside the housing.
Hence, in the active antivibration device disclosed in Japanese Patent Publication No. 2008-208895, it is required that a fixing portion of the disc spring member to be fixed to the housing is set at a position outer than the outer periphery of the output member. As a result, an outer diameter of the housing becomes large, thereby resulting in difficulty to decrease the outer diameter of the housing.